In many animal species, germ cell fate determination depends on the maternal inheritance of the germ plasm, a specialized cytoplasm composed of ribonucleoparticles (RNPs) and enriched in cytoskeletal elements. Germ plasm RNPs are typically inherited as particles within the egg, which during early embryogenesis become aggregated into large masses. These become segregated into primordial germ cells (PGCs), where they are thought to promote the initiation of the germ cell transcriptional program. In zebrafish, as in a number of other vertebrate species, gathering and segregation of germ plasm RNPs is coupled to cell division. Zebrafish germ plasm has been shown to be required and sufficient for germ cell induction in this organism. We have previously delineated specific steps during cell division that result in germ plasm RNP recruitment to the furrows for the first several cycles, with subsequent compaction at the furrow distal ends. These steps entail a gradual increase in germ plasm RNP aggregation and result in the stabilization of four large germ plasm masses. These masses are inherited by PGCs, to eventually disperse through the cytoplasm during PGC specification. Our overarching hypothesis is that germ plasm RNP aggregation is mediated by adaptations of the cellular machinery, relying on the interaction between a dynamic, excitable cortex and fluid phase-like properties of RNPs and associated intrinsically disordered proteins (IDPs). In Aim 1, we will address how the regulation of an excitable actomyosin network mediates germ plasm RNP movement and aggregation, and will study the function of factors components of the Chromosomal Passenger Complex (CPC) in cytoskeletal network dynamics and germ plasm RNP anchoring. In Aim 2, we will study the behavior of segregating germ plasm RNPs as ordered supramolecular structures at the furrows, as well as the role in germ plasm segregation of the previously uncharacterized IDP Bucky ball 2-like. The germ cell fate shares characteristics with pluripotency and cancer, and our studies will provide new knowledge applicable not only to reproduction but also cell reprogramming and cancer biology. Understanding of membrane-less compartmentalization through fluid phase behavior will provide additional fundamental insights relevant to human biology and disease.